Researchers crunch numbers on African Horse Sickness export risk

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The risk of a horse infected with African Horse Sickness (AHS) arriving undetected in a country after being properly quarantined in a nation where the disease is endemic is extremely small, research suggests.

AHS is an often fatal viral infection transmitted by biting Culicoides midges. It occurs in most of sub-Saharan Africa and is a significant impediment to the export of live horses from infected countries, such as South Africa.

South Africa has defined disease-free areas and surveillance zones, surrounded by a protection zone, which enables it to export horses.

Evan Sergeant, John Grewar, Camilla Weyer and Alan Guthrie developed a model to estimate the chances of exporting an undetected infected horse through a vector-protected pre-export quarantine facility operated in accordance with World Organisation for Animal Health (OIE) recommendations for trade from an infected country.

Their model allows for additional risk management measures to be factored in where appropriate, including multiple DNA-based PCR tests before and during pre-export quarantine, and optionally during post-arrival quarantine. It can also be used for comparing the risk associated with exports from a demonstrated low-risk area for AHS and an area where the disease is endemic.

Sergeant and his colleagues, writing in the peer-reviewed open-access journal PLOS ONE, calculated that if 1 million horses were exported from a low-risk area with no post-arrival quarantine, the average number of infected horses would be 5.4. Assuming 300 horse exports a year, this equated to an annual probability of 0.0016.

Their modelling indicated that an additional PCR test while in vector-protected post-arrival quarantine reduced these probabilities by about 12-fold.

The probabilities for horses exported from an area where the disease is endemic were about 15 to 17 times higher than for horses exported from the low-risk area under comparable scenarios.

The study team noted that the OIE code recommended the use of vector-protected quarantine and testing as an appropriate measure for risk management for export of live horses from infected countries.

“However, few countries have been prepared to accept exports from South Africa under these guidelines, other than via the free zone, presumably because of the perceived risk associated with the possible breakdown of vector protection and lack of back-up risk mitigation measures.

“The results presented here clearly demonstrate that it is possible to manage the risk of AHS infection in horses exported from an infected country or zone,” they wrote.

“The probability of exporting undetected infected horses could be further reduced, effectively to zero, by limiting exports to periods when there are no outbreaks.

“This would be achievable in the low-risk area and would still allow exports to proceed unimpeded in most years and for the majority of the year when an outbreak does occur. However, this might be more difficult for exports from the endemic area where, on average, outbreaks occur in nine months of any given year.

“The occurrence of undetected outbreaks in the low-risk area would result in an increase in the probability of exporting an undetected infected horse. However, there is a high awareness of AHS in the low-risk area, along with movement controls and active and passive surveillance, so an outbreak is highly unlikely to remain undiscovered for more than a short period of time …”

They concluded: “The results of this research clearly demonstrate that the risk of AHS infection in horses exported from an infected country can be minimised by appropriate risk management measures.

“Critical components of risk management for AHS are a vector-protected quarantine facility for pre-export quarantine, supported by pre-export testing using recommended tests for AHS.

“However, additional risk management may be required to provide backup in case of a breakdown in vector protection, in the form of either pre-border measures, based on a demonstrated low risk of infection prior to and during pre-export quarantine, or post-border by the use of vector-protected post-arrival quarantine and additional testing.”

The final choice of risk management measures depended on the level of risk acceptable to the importing country, they said.

The researchers are variously affiliated with the University of Pretoria and the Western Cape Department of Agriculture, both of which are in South Africa; and AusVet Animal Health Services in Australia.

Sergeant ES, Grewar JD, Weyer CT, Guthrie AJ (2016) Quantitative Risk Assessment for African Horse Sickness in Live Horses Exported from South Africa. PLoS ONE 11(3): e0151757. doi:10.1371/journal.pone.0151757

The full study, published under a Creative Commons License, can be read here

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